I found a thread similar to this but it didnt help me...
When i get to the elevator and activate it we start descending...
then that B**** Alyx says look up watch out...
the debris start falling and all of the sudden lightning comes and destroys the elevator...
WTF???!?!?!??!?!?! please help me
EDIT: Problem Solved! i used the command from here

quote muted rabbit

im having trouble playing half life 2 ep. 1 on my pc. i have a pretty good system (3.2 ghz intel pentium D processor, 1 gb of ram, ati radeon x550) and it seems that hf2ep1 ends up showing my transparent objects (med kits, car (in the first level), the broken pipe (the one u have to duck under in the first level)and etc). i've noticed in the console a whole list of messages in red says things like this “error vertex file for ‘name of files.mdl’ not found”

Here are the solutions i've tried...
-updating my driver
-downloading and installing the newest version of directx9
-re-installing hf2ep1
-(also i looked this up on another forum) typing 'mat_dxlevel 90' in the console which works but it weighs REAL heavy on my computer so im forced to tone down the graphics a lot.

if anybody has a clue as to how i may play this gorgeous game smoothly (keep in mind the invisible object issues) without having to sacrifice the stunning visuals, it would be greatly greatly appreciated

It's no bug...you should use gravity gun to prevent the large debris falling on elevator. When those huge chunks are close enough, shoot them with the only weapon you've got!
Hope it's helpfull